Regulation Of Polyamide Layer For High-performance Nanofiltration Fabrication And Performance Evaluation

At present,water shortage and water pollution have become a global challenges.Membrane separation,as an innovative technology to remove pollutants via physical retention without the addition of chemicals,is used in water pollution control and drinking water purification.Nanofiltration membranes are widely used in surface water purification,brackish water treatment and wastewater reuse due to their low operating pressure and high permeance,which can effectively retain dissolved organic matter and hardness.Thin film composite membranes consisting of a polyamide layer and a porous substrate are the commercially available nanofiltration membranes.However,due to the trade-off between membrane permeation and separation ability,the existing nanofiltration membranes exhibit low permeance to maintain the essential salt rejection rate,requiring a larger membrane area and greater operation pressure in practical application.The membrane fouling is another primary obstacle,resulting in the frequent chemical cleaning.This will shorten membrane lifespan.The polyamide layer dominates the mass transfer resistance,separation performance and fouling resistance of nanofiltration membranes.Therefore,in this paper,the polyamide layer is regulated and optimis ed from the interface design perspective,and several nanofiltration membranes with high flux and anti-fouling resistance are prepared.The rational design of a ceramic-based nanofiltration membrane remains a significant challenge due to its performance and fabrication cost.Herein,we report a high-performance ceramic-based nanofiltration membrane fabricated via a typical interfacial polymerization on an interwoven net substrate assembled by titanium dioxide（Ti O2）nanowires.The chemical properties and morphologies were systematically investigated for ceramic substrates and their corresponding nanofiltration membranes.Due to the significantly improved hydrophilicity of the Ti O2 framework,more reactive amine monomers were uniformly adsorbed on the modified surface of the ceramic substrate,yielding an ultrathin polyamide layer with less resistance.In addition,the smooth surface and decreased pore size of the Ti O2 framework contributed to forming a defect-free polyamide layer without intrusions into the substrate.As a result,the obtained ceramic-based nanofiltration membrane evinced high permeance of 26.4 L·m^-2·h^-1·bar^-1 and excellent salt rejection efficiency,leading to simultaneous improvements compared with the unmodified nanofiltration membrane without the Ti O2 framework.Notably,the potential regeneration ability of the ceramic-based nanofiltration membrane could be achieved via facile low-temperature calcination and re-polymerization process due to the varied thermostability between the polyamide layer and the robust ceramic substrate.The operation of regeneration helped to prolong the lifetime and decrease the cost of the ceramic-based nanofiltration membrane.This research provides a feasible protocol for fabricating sustainable ceramic-based nanofiltration membranes with enhanced performance for water treatment.To further improve the permeance of the membrane,a nearly nonporous two-dimensional lamellar layer was formed on the surface of the substrate using transition metal carbides（MXene）.The MXene layer showed a smoother surface,higher hydrophilicity and more absorption of reactive monomer,while also endowing the resistance for the substrate.The polyamide layer was subsequently fabricated by a classical interfacial polymerisation on the MXene layer.The higher concentration of amine monomer promoted the self-containment and self-termination process of interfacial polymerisation,reducing the thickness of the outer polyamide layer.The almost nonporous lamellar interface inhibited the formation of the inner polyamide layer within membrane pores.In addition,through a mild oxidation reaction,the MXene nanosheets were oxidised to 20 nm Ti O2 nanoparticles,which subsequently passed thro ugh the pores of the PES substrate,thereby eliminating the resistance of the MXene layer.The prepared nanofiltration membranes exhibited over 96%rejection of Na2SO4 and permeance of 45.7 L·m^-2·h^-1·bar^-1,which was 4.5 times higher than that of the unmodified nanofiltration membrane.This study fully analysed the effects of the inner and outer polyamide layers on the mass transfer resistance of nanofiltration membranes,and provided a theoretical basis for the preparation of high flux nanofiltration membranes.In response to the challenge of the nanofiltration membrane fouling,N-isopropylacrylamide（NIPAM）was grafted onto the bromine-containing polyamide layer.The modified nanofiltration membrane had a rougher surface,greater hydrophilicity,lower membrane charge and larger membrane pore size.Due to the reduced thickness of the polyamide lay er,the modified membrane had high water permeance of 16.8 L·m^-2·h^-1·bar^-1.In addition,the nanofiltration membrane showed similar removal of divalent ions.PNIPAM chains yielded a better deposition resistance and adhesion resistance,thereby mitigating the increase of fouling and promoting the recovery of flux during the filtration and traditional cleaning stages,respectively.Moreover,PNIPAM chains shrank when the water temperature was above the lower critical solution temperature（LCST）,indicating the formation of a buffer layer between the membrane and the pollutants.The buffer layer would eliminate the membrane-foulant interaction energy,thus further enhancing the detachment of pollutants.The simple and efficient cleaning method could act as an enhanced cleaning procedure to remove irreversible fouling.This provides new insights into the fabrication of enhanced antifouling membranes by using smart responsive polymer chains.